The International Atomic Energy Agency (IAEA), the key multilateral global nuclear governance body, describes itself as the “global platform” for nuclear security efforts, with a “central role” in facilitating international cooperation in the field. Long concerned with the physical protection of nuclear materials and facilities, the Agency began to ramp up its involvement in the broader issue of nuclear security after the 9/11 terrorist attacks. The series of Nuclear Security Summits, which ran from 2010 to 2016, drew high-level political attention to the threat of nuclear terrorism for the first time and boosted support for the IAEA’s nuclear security mission. The final summit, held in Washington, DC, in March 2016, lauded the Agency as “crucial for the continuing delivery of outcomes and actions from the nuclear security summits.” Participating governments agreed to a seven-page “Action Plan in Support of the International Atomic Energy Agency.” Three years after the final summit seems an opportune time to assess how the Agency’s nuclear security work has fared since then. Given the complexity of the Agency’s nuclear security activities, this paper cannot provide a comprehensive assessment, but will highlight the most important nuclear security activities and the constraints and challenges the IAEA faces in fulfilling its nuclear security role.